1. Field of the Invention
The present invention relates to an image processing apparatus, an image display apparatus, an image processing method, an image display system, and an image processing system. The present invention also relates to a program for causing a computer to execute such an image processing method, and a computer-readable storage medium storing such a program.
2. Description of the Related Art
Recently, with image input apparatuses such as digital cameras and scanners becoming popular, it has been more commonly practiced to store digital image data in a storage device such as a memory or a hard disk of a personal computer (PC) or in an optical disk such as a CD-ROM, or to transmit digital image data via the Internet. Such digital image data is compressed and coded to be stored in the storage device or the optical disk.
However, in the case of decompressing and rendering a compressed code on a display unit having a rendering region considerably small in comparison to the size of the original image, such as the display unit of a PC, a personal digital assistant (PDA), or a mobile terminal, the image cannot be rendered before the entire compressed code is decompressed. This causes a problem because it takes a large amount of time before the image is rendered on the display unit.
There have been several proposed conventional methods to solve this problem.
According to one of the methods, in the case of decompressing a horizontally continuous panoramic image, the frame is divided horizontally in a plurality of frame parts, so that the frame parts are decomposed one after another from the one to be rendered earliest in time. According to this decompression method, however, the frame should also be divided vertically in the case of using an input device having high resolution in two-dimensional directions, such as a high-resolution scanner. Otherwise, if the image becomes greater in size vertically than the rendering region, there would be a problem that compressed code that has been coded according to a coding method performing no vertical division cannot be decompressed at high speed or that work memory is consumed more than required.
According to another method, a JPEG codestream is decompressed continuously from its beginning, and the decompression result is managed by the line in accordance with the size of the rendering region. According to this method, decompression can be performed at high speed by the effect of the high-speed decompression characteristic of JPEG itself. However, it is always necessary to decompress the codestream from its beginning in order to decompress data in the end part of the codestream. Accordingly, the decompression result of the leading part of the codestream is discarded. Thus, there is room for improvement left in this method in terms of the effective use of CPU resources and the high-speed decompression of the end data of the codestream.
According to yet another method, in the case of rendering a large image such as a map stored on an optical disk, each image is divided into unit-size images, and the unit-size images necessary for rendering the image are successively read out as disclosed in Japanese Laid-Open Patent Application No. 63-92988 (Patent Document 1). According to this method, when each image is divided into unit-size images, and the unit-size images necessary for rendering the image are successively read out, toroidal memory structures in which the upper and lower ends and the right and left ends of memory spaces are connected as link addresses are coupled as essential components. This not only complicates the control, but also requires a memory capacity area twice as much as that of the rendering memory, thus resulting in high costs.
Under these circumstances, coding methods that divide a large image into two-dimensional smaller blocks called blocks by a method called block division and decompress each block independently as disclosed in Japanese Laid-Open Patent Application No. 2000-36959 (Patent Document 2) have been proposed recently.
However, even in the case of decompressing a compressed image coded by the coding method disclosed in Patent Document 2, a display unit having a rendering region considerably small in comparison to the size of the original image takes a lot of time, for the area of its rendering region, to render the image. Therefore, the coding method of Patent Document 2 provides no fundamental solution to the problem.